1. Technical Field of the Invention
The present invention generally relates to optical networks. More particularly, and not by way of any limitation, the present invention is directed to a lightpath segmentation system and method for application in a Generalized Multi-Protocol Label Switching (“GMPLS”) network for facilitating fair and effective reservation of lightpaths therein.
2. Description of Related Art
In all-optical GMPLS wavelength division multiplex (“WDM”) networks, lightpaths, or label switch paths (“LSPs”), may collide and fail during the lightpath establishment procedure. This is referred to as the wavelength collision problem. Shorter lightpaths have an advantage over longer lightpaths in reserving wavelengths. FIG. 1, which depicts a block diagram of an optical network 100, illustrates why this is the case.
In particular, assuming for the sake of example that a first LSP Request (“LSP Request 1”) requesting establishment of a first lightpath 102 between an ingress node 103a and a egress node 103b, and a second LSP Request (“LSP Request 2”) requesting establishment of a second lightpath 104 between a ingress node 105a and a egress node 105b, are from their respective ingress nodes along their respective paths at the same time. Because the lightpath 104 is shorter, LSP Request 2 will be received at the egress node 105b before LSP Request 1 is received at the egress node 103b. Consequently, the lightpath 104 will be established first.
This may also be the case in a situation where the LSP Request 1 is issued before the LSP Request 2, given that lightpath 102 is significantly shorter than lightpath 104. If both lightpaths 102, 104, reserve the same wavelength, a collision, referred to as a “reverse collision”, will occur at the node 105b and appropriate error messages will be sent to nodes 103a and 103b, indicating that a different wavelength must be reserved for the lightpath 102.
In existing protocols for lightpath reservation in GMPLS, an optical switch controller (“OSC”) at each node in an optical network maintains two “pools” of wavelengths, including an “Available Pool” (“AP”) and a “Used Pool” (“UP”). The AP includes wavelengths that available to be reserved by a lightpath. The UP comprises wavelengths that are currently being used by existing lightpaths.
Currently, in the GMPLS signaling control plane (RSVP/LDP/CR-LDP), upstream nodes insert a “Label Set” object into each LSP Request (e.g., Path) message. The Label Set object suggests the labels (which correspond directly to wavelengths in the optical domain) that can be selected by the downstream nodes. In other words, the Label Set represents the wavelengths available for selection as indicated in the AP. Downstream nodes select one of the labels, or wavelengths, in the Label Set as specified in the Label Set object.
In general, at each node along the path during LSP establishment, the OSC at the node compares the wavelengths of the Label Set object with those in its UP and removes from the Label Set object any common wavelengths. Similarly, once a wavelength has been reserved by a lightpath, the reserved wavelength is removed from the AP and included in the UP maintained by each node along the path.
FIG. 2 illustrates lightpath establishment procedures in a GMPLS network 200. The GMPLS approach, with RSVP control plane, establishes a lightpath 201 between an ingress node 202 and an egress node 204 via intermediate nodes 205a–205d using Path messages, such as a Path message 206, and Resv messages, such as a Resv message 208. The Path message 206 includes a Label Set object that defines a Label Set comprising a list of the wavelengths available for selection at each node 205a–205d along the lightpath 200. As described above, and as illustrated in FIG. 2, at each node along the lightpath 201, the Label Set contained in the Path message 206 is examined and wavelengths that are in use at that node are removed therefrom.
Accordingly, a first Label Set 210a between the ingress node 202 and a first intermediate node 205a larger than subsequent Label Sets 210b, 210c, 210d, and 210e, between subsequent node pairs 205a/205b, 205b/205c, 205c/205d, and 205d/204, respectively, such that the Label Set 210e that reaches the destination, or egress node 204, may be relatively small compared to the initial Label Set 210a. Clearly, some of the wavelengths contained in the Label Set 210e may be in the Used Pool of the egress node 204, leaving even fewer available wavelengths from which a wavelength may be reserved for the lightpath 201. A situation in which there are no available resources at the destination node is referred to as a “forward collision”.
Accordingly, while the above-described prior art techniques address the wavelength continuity problem, they fails to adequately address the short lightpath/long lightpath disparity described above with reference to FIG. 1. Moreover, they fail to solve the reverse collision problem. In particular, present techniques do not prevent the suggestion of the same wavelengths to, and the possible selection of the same wavelength by, more than one LSP. For example, referring again to FIG. 1, assuming LSP Request 1 is issued at a time t1 and LSP Request 2 is issued at a time t2 after time t1. Assuming further that LSP Request 2 is received at the node 105b at a time t3 and LSP Request 1 is received at the node 103b at a time t4 after time t3. If LSP Request 1 is received at the node 105b any time before time t3, the wavelength that will ultimately be assigned to the lightpath 104 will still be in the AP at that node; therefore, it is conceivable that the same wavelength will be assigned to both paths 102 and 104.
Additionally, present techniques provide no solution to the forward collision problem, as illustrated in FIG. 2. Wavelength conversion may reduce or eliminate both forward and reverse collision problems; however, wavelength converters are expensive, rendering it impractical to include such equipment at every node in a network. In order to provide a reasonably-priced optical switches, the number of wavelength converters should be limited and optimized.